1. Field of the Invention
The present invention generally relates to a monolithic microwave integrated circuit (MMIC), and more particularly to a microwave double-pole-three-throw switch, used for WLAN and Bluetooth dual-mode co-existence system.
2. Description of the Related Art
The rapid development of the co-existence operation of multi-standard wireless and mobile communication has been driving conventional radio frequency (RF) and baseband transceivers to have integrated multi-band and multi-functional characteristics, such as the multimode wireless local-area network (WLAN) IEEE802.11a/b/g card, the integrated WLAN/Bluetooth card, and the integrated GSM/WLAN handset. The IEEE802.11b standard operates in the frequency band of 2400-2483.5 MHz, which provides the transmission rate of 1-11 Mbps. The IEEE 802.11g standard, operating with the same 2400-2483.5-MHz band, has higher transmission rates, up to 54 Mbits per second. On the other hand, the wireless personal network (WPAN), such as Bluetooth and IEEE 802.15.4 standards, operates at the same 2400-2483.5 MHz and provides the feature of extremely low power consumption for low data transmission rates. For the performance and product-value enhancement of wireless products, such as notebook and PDA, the incorporation of WLAN and Bluetooth into the original product becomes crucial.
FIG. 1 shows a conventional schematic of a IEEE802.11b/g WLAN and Bluetooth co-existence system. In general, the system employs three antennas, where two antennas are used for IEEE802.11b/g and the third antenna is for Bluetooth system. The two WLAN antennas are followed with a double-pole-double-throw switch and the Bluetooth antenna is followed with a single-pole-double-throw switch for transmit and receive operation. The WLAN antenna has an antenna diversity effect while the Buletooth antenna does not have the antenna diversity effect.
On the aspect of circuit design, the field-effect transistors or diodes are used as the switching elements, which are arranged in various structures. First, the series-shunt architecture or their higher-order extensions, such as the series-shunt-series T-type structure or the shunt-series-shunt Pi-type structure. This structure has low RF power handling due to the voltage drop all across the transistor nodes, causing voltage breakdown. It also suffers poor insertion loss due to the turn-on resistance of the transistor and poor isolation due to the parasitic drain-source or collector-emitter capacitance. Second, the L-C resonant structure uses additional inductor to resonate the parasitic drain-source or collector-emitter capacitance and release the voltage drop across the transistor nodes. The paper, reported by Tokumitsu et al, entitled “Low Voltage, High Power T/R Switch MMIC Using LC Resonators,” IEEE Microwave and Millimeter-Wave Monolithic Circuit Symposium, pp. 27-30, June, 1993, provided a novel T/R switch for high-power/low-distortion operation at low control voltage. The LC-resonant switch structure, composed of inductors, capacitors, and switching FETs, is incorporated in the TX and RX circuit path for high power handling and low insertion loss. An LC-resonant T/R switch with total periphery of 2.88 mm exhibits the third-order inter-modulation ratio higher than 40 dB for input power up to 28 dBm when controlled at 0V and −2V.
Another paper, reported by Tokumitsu et al, entitled “A low-voltage, high-power T/R-switch MMIC using LC resonators,” in IEEE Transactions Microwave Theory and Techniques, vol. 43, pp. 997-1003, May, 1995, provided a novel T/R switch for high-power/low-distortion operation at a low control voltage. A 1.9-GHz LC-resonant T/R switch MMIC with a total FET periphery of 3.36 mm exhibits third-order inter-modulation ratio higher than 40 dB for input power up to 31 dBm when controlled by a single-polarity voltage −2 V. This MMIC occupies an area less than 2×2 mm2, which makes it possible to implement advanced transmit/receive switches for applications in PCS and ISM frequencies below 5 GHz.
U.S. Pat. No. 5,990,580 issued to Weigand et al, entitled “Single-pole-double throw switch”, discloses an electronic SPDT switch. The electronic SPDT switch has a series field-effect transistor (FET) in a first circuit arm between a common port and a first port, a shunt FET in a second circuit arm between the common port and a second port, the shunt FET being isolated 90 degrees or ¼ wavelength from the common port, a source applying pull-up voltage to sources of respective FETs and to the common port, to provide a connection of the common port with the second port, a source applying a first control voltage of opposite logic state to the gates, and the FETs being in the depletion mode that conduct at a zero sum of the bias voltage and the control voltage, and that conduct when DC power is interrupted, to provide a connection of the common port with the first port. However, in the above technologies, the single-pole-double-throw switch can not meet the requirement of the IEEE802.11b/g WLAN and Bluetooth dual-mode co-existence operation.
According to the above problems, there is a need to provide an antenna diversity switch to overcome the above problems, meeting the requirement of IEEE 802.11b/g WLAN and Bluetooth co-existence operation with the reduction of three or four antennas in prior art to two antennas. Moreover, the antenna diversity for both WLAN and Bluetooth modes are also provided.